Circuit Information

Please see this Picture for an illustration of some soldering tips. Use the smallest soldering tip you have. The soldering pads are small and a large tip will cause trouble. Be very careful with a large tip and check for solder bridges to ground.

For detail information on the theory and design of the frequency counter, check out Neil's article "A PIC-Based Digital Frequency Display", May 1997, QST, pp 36-38. Topics covered are Theory of Operation, Setting up the DFD, and Operating Tips.

For more detailed information on these counters, check out Neil's web site: http://www.aade.com

The instructions for the DFD2 that uses a 10 MHz or 16 MHz crystal for the oscillator are located at DFD2 10 & 16 MHz Instructions. After building the unit return here and start at Testing

If you have the 20 MHz TXCO module, which is currently being shipped with the kits, follow the instructions below.

AADE DFD2 w/TXCO Instructions

There a 3 general steps to complete assembly and packaging of the DFD2.

1. Choose the final installation method. The suggested location is on the bottom blank PCB board. If you are building a case for the receiver, it is suggested to move the counter at least 3" from the receiver boards to help prevent counter birdies getting into the receiver. The optional
back lighted display is included with the kits that are purchased with the receiver.

Printed Circuit Board Assembly

Begin the actual PCB assembly by sorting out the supplied components. A Parts List is included with the kits shipped with the receiver, including additional parts for automatic switching of the bands.

Assemble the PCB per the following illustrations:

Step 1

Check the back of the board to make sure the TXO has been pre-soldered to the PCB. Neil has been shipping all the new kits with the TXO already soldered to the board.

Step 2

The parts are mounted with the ground plane of the board facing up.

Mount the IC sockets and the display header. Note the notches on the sockets. They are pointed out with the yellow arrows in the above picture.

The long pins on the header are mounted up. Short pins go into the PCB.

Step 3

Install and solder all resistors and the six back to back diodes.

A yellow line has been placed on the diodes to show where the band should be placed.

Double check the bands on the diodes before soldering. The diodes are very difficult to remove and reinsert.

Step 4

Install and solder the 9 .1 mfd capacitors.

Step 5

Install the 78L05 voltage regulator, the 10 mfd electrolytic and the two 15 turn trimpots.

The positive side of the 10 mfd electrolytic is located in the hole located closest to the edge of the PCB. The negative side of the capacitor is marked on the part.

Mount the flat side of the 78L05 toward the IC sockets as shown in the picture above. Be sure to mount it down as far as it will go on the board. Don't leave it sticking up in the air.

Step 6

On the back side of the PCB install and solder the 10K contrast control, the 25K calibration control, and the two 2 pin headers.

The short pins on the 2 pin headers go into the PCB, long pins face up.

Step 7

Install the PIC16C71 and the 74HC153.

On the far left, the PIC16C71, an 18 pin part.

Next on the left, the 74HC153, a 16 pin part.

The three horizontal chips are the 74HC4046s. Do not install them now. They will be installed later during "Testing" and "Setting the VFO Frequencies".

Note the notch location in the picture above and double check after you have placed them in the sockets.

Step 8

Install the female header on the display module. Note that the header is installed on pins 1-14. Be sure that pin 15 and 16 are empty! See picture above.

When mounting the display module on the PCB, note that the 10 mfd capacitor may need to be tilted over.

Make sure the header is mounted straight up on the display module. See picture above.

Solder one pin on the header, then reheat to get it positioned straight. Recheck position, then solder all the pins.

Testing

____Carefully inspect all of your solder connections, the polarity of the diodes, voltage regulator, the electrolytic capacitor, and the notch of the ICís.

____Adjust the contrast control fully counter-clockwise.

____Plug the display module into the PCB 14 pin header.

____Attach the power supply, 8 to 12VDC, or 9V battery leads, to the marked pads on the back of the board near the bottom. Be sure to observe polarity with the negative lead to the
grounded pad. (Picture)

If you reverse the power connections, the 78L05 will smoke. Not to worry, after replacing the 78L05, the counter should work.

____You should see some frequency displayed. Adjust the contrast control for the desired effect.

Wiring the LED back-lit display

The back-lit module has a block of LEDs behind the LCD display. It is powered from pins 15 and 16 with pin 15 being plus and pin 16 being minus (usually
ground). The voltage drop across the LEDs is 4VDC. The current can range from 20ma to 150ma. The dropping resistor required is R = (V-4)/I where I is the
desired current and V is the supply voltage.

One method that works well is to power it from the 78L05 regulator on the DFD2 as follows:

____On top of the display module, solder an insulated jumper wire from pin 1 (ground) to pin 16 (or the top connection of the LED module on the right side).

____Solder a 33ohm 1/4 watt resistor from pin 2 (+5VDC) to pin 15 (or the bottom connection to the LED module on the right side). The display is about as bright as a car radio dial at night.

Preparing the cables and soldering to the Frequency Counter PCB

____Solder the cable prepared for the VFO to the VFO input (middle set of pads) on the counter PCB.
____Solder the cable prepared for the Crystal Oscillator to the HFO input (bottom set of pads).

____The BFO input will not be used with the receiver. Do not install the 74HC4046 in the top socket.

Or use a small piece of bare wire and short the BFO input to ground (directly on the PCB) and solder the jumper (Not necessary if the chip is not installed).

Setting the VFO Frequencies

If you do not have a bench Frequency Counter and need to use the DFD2 to set the VFO frequencies, follow these instructions. If you are using a bench counter to set the VFO, jump to Setting the 455 kHz Offset

____Remove all the 74HC4046s from the counter PCB.

Or,

____Short together the center conductor and shield that will connect to the Crystal Oscillator.
____Short together the center conductor and shield that will connect to the VFO.

____Apply power to the frequency counter and set the coarse (top pot) and fine (bottom pot) adjustments to show zero frequency on the display. Turn the pots counter-clockwise to set to zero.

Sometimes the counter will show all squares and no digits. Disconnect and re-apply power and the digits should show. Also, turn the viewing angle of the display to make sure they are visible.

____Disconnect power to the display.
____Install a 74HC4046 in the middle socket. This will allow VFO energy to reach the counter. (Or remove the short on the VFO cable, leave the HFO cable shorted.)
____Attach the VFO cable from the frequency counter PCB (center pair of pads) to the "Freq Cntr" box at the output of the VFO oscillator.
____Turn on the power to the receiver and the Frequency Counter.

Be very careful of reverse polarity connections. A reverse polarity connection to the frequency counter will blow the voltage regulator (78LO5). Use a diode at the 12 Volt input to the Frequency Counter if you think you might make a mistake. Band side of the diode goes to the outside where you will be attaching power.

____The Frequency Counter will display the VFO frequency.
____Set the 2/8 ceramic trim pot (labeled 14.068) to its middle position. Line up the slot in the adjustment screw with the capacitor pins, with the rotor plate on the end with the arrow. The rotor plate is the part of the top with the metallic color.
____Spread or compress the VFO toroid coil to set the frequency as close to 14.068 as possible.

Note: All tuning to make the frequency reach 14.068 is done by modifying the turns on the toroid.

If the frequency is too low with the turns moderately spread apart (13 MHz to 13.5 MHz), remove one turn on the toroid.

If the frequency is too high, add a turn to the toroid. (This may require rewinding the toroid for a clean appearance, but having the frequency read too high, >15MHz, is unlikely.)

____Secure the VFO toroid by whatever means you have decided. Some options are a small plastic wrap, a 4-40 screw/nut and 3/16" rubber grommet (cut to fit into the center of the toroid), or pour wax from a candle (let wax cool before proceeding). Picture

____Re-adjust the 2/8 ceramic trim pot labeled "14.068" to 14.066 MHz (Gives some room at the low end of the band.)
____Set the receiver to 40 Meters. Bandpass switch to the 40/30 side and the Crystal Filter switch to the 40/20 side. One LED should be on next to the 10.545 relay.
____Adjust the yellow trimmer next to the 10.545 relay for a frequency between 10.540 MHz to 10.543 MHz. (Gives some room at the low end of the band.)
____Set the receiver to 20 Meters. Bandpass switch to the 20/17 side and the Crystal Filter switch to the 40/20 side. Two LEDs should be on.
____Adjust the yellow trimmer next to the 10.445 relay for a frequency between 10.440 MHz to 10.443 MHz. (Gives some room at the low end of the band.)

Setting the 455 kHz Offset on the Counter

____Remove the VFO input 74HC4046 IC (middle chip). All the 74HC4046 chips should be removed for this adjustment.

____Make sure the reading is 0 kHz. This is done by turning the trimpots all the way counter-clockwise.

____Turn the coarse pot (upper one) clockwise to 384 kHz.

____Turn the fine pot (lower one) clockwise to reach 455 kHz.

Automatic Switching

The offsets and the adding/subtracting are done at Pins 12 and 13 of the PIC16C71. Pins 12 and 13 are brought out to a four pin set on the back of the counter. The chip has 100K internal resistors that bias the pins to 5 volts.

Two of the pins are grounds and the offsets are normally set by using a jumper to ground the necessary pins. With this receiver, wires are soldered to the left side pins (looking at the back). The right side pins are grounds. The left side pins are labeled "upper jumper" and "lower jumper" in these instructions.

There are two ways to connect the Frequency Counter to the receiver to achieve automatic switching of the bands on the counter. One is by using a small interface unit that uses diodes and two NPN transistors that connect to the switches on Board 1 of the receiver.

The second one is by using a rotary switch. The easiest one to wire is a two section rotary, either one wafer with two sections or two wafers. A rotary switch with one wafer can be used but is a little more complex to wire.

Connecting the Frequency Counter to the Receiver

Note: The "Frequency Counter" output between the VFO amplifiers will not be used to drive the Frequency Counter. This connection overdrives the Frequency Counter and causes birdies in the receiver.

____Attach the HFO cable from the Frequency Counter (bottom pair of pads) to the Crystal Oscillator output box labeled "Freq Cntr". Picture
____Install the middle and bottom 74HC4046s. Leave the top one empty. The top one is the BFO input and will not be used.

Mounting the Counter on the Receiver

Getting the birdies of the counter out of the receiver has been researched for many hours. Just recently an accidental discovery showed the best way to mount the frequency counter for minimum birdies into the receiver.

Almost all the noise from the counter comes from the LCD updating circuit and is radiated by the display bezel and the display itself. Normally one would think that using good grounding techniques would eliminate that noise.

However, when adjusting the counter frequencies one day, the display was insulated from ground and there was no display noise in the receiver! A ground loop with the display must make the noise float in the ground circuit of the receiver.

Therefore, when mounting the frequency counter, DO NOT ground the display, let it float! The only ground to the display should be at the back of the PIC PCB ground connection on the back of the counter.

Since the mounting holes for the frequency counter are in the display board, this presents some extra effort to mount the display without grounding it.

Tito Magluyan, AC5QC presented the following idea on mounting the frequency counter. Using the teflon screws was added on later.

Using teflon screws and nuts are the easiest way to insulate the display. Tito used 12-2-G house wiring to make the mounting hardware.

Turns out #12 solid copper wire scrapped from house wiring fits very nicely inside the mounting holes of the display. Cut a couple of small pieces, bend one end around to make a place to mount a screw, then bend the other end to go through the bottom mounting hole of the display.

Take a teflon screw and run it through the bottom of the board and secure with a nut. Mount the display with the #12 copper wire mounting holes, and then take a second nut to secure the display.

The picture below shows how it is done a lot easier than trying to explain the process:

Yellow arrows point to the #12 house wiring that is used to help mount the display. The #12 solid copper wire just fits inside the mounting holes of the display and are soldered to the display PCB.

In case teflon hardware is not available, a trip to the hardware store yielded some 1/8" nylon wire clamps found in the wiring section.

A slot is cut in the middle of the nylon clamp for the PCB board to slip into.

Cutting the middle of the nylon clamp with a razor blade can be dangerous as the nylon is very hard. I used a hack saw blade to make an initial cut in the middle of the clamp, then a razor blade was used to make the slot wide enough for the PCB board. A pair of pliers (or a pair of vice grips, or a small vice) is used to hold the nylon clamp.

Then a tie wrap (the smallest available, approximately 1/8" or smaller) is use to mount the nylon clamp to the display mounting holes.

Picture shows both sides with the modified nylon clamps. Hardware (4-40 screw and nut) is used to hole the clamp together for easier handling.

Picture shows the final product with the wire wraps cut.

Final mounting of the display.

The picture above shows a shield mounted around the counter. The shield is not grounded to the display.

A small shield should be put around the counter to eliminate the remaining noise. One that just covers the counter between the top of the display and the bottom board of the receiver pretty much leaves a clean receiver.

Single sided blank PCB shield material is added to the kit for this purpose. Tin snips prove to be the best way to cut this material. You can also scribe a line on both sides with a sharp razor blade or knife and break apart the pieces you need.

____The LED at the 10.545 text will be on for 40 Meters. The counter should read 7.000 MHz (or slightly below - 6.995 to 6.998 MHz).

Twenty Meters

____Set the Crystal Filter Switch to the 40/20 position.
____Switch the Bandpass Filter to the 20/17 position. This will put you on twenty meters.
____Both VFO LEDs will be on for 20 Meters. The counter should read 14.000 MHz (or slightly below - 13.995 to 13.998 MHz).

Circuit Description/Input Circuit

DFD2 is intended for the type unit consisting of a crystal controlled converter in front of
a tunable IF. Connections must be made to the High Frequency Oscillator (HFO), the
Variable Frequency Oscillator (VFO) , and optionally to the Beat Frequency Oscillator (BFO).

It then computes the carrier frequency of the signal as RF = HFO +/- VFO +/- BFO. In AM
mode the BFO should be zero in which case if displays RF = HFO +/- VFO +/- IF, where
IF is set by the offset trimpots or a stored value.

All inputs are protected by the transient stopper interface shown below. This is intended
to stomp transients that occur when switching bands in your radio.

It may not be good to allow this circuit to clip the input sine wave. Depending on the source
impedance of the signal it may distort the sine wave, generating spurious frequencies.
To attenuate input signals, tack solder a resistor in parallel with the diodes (on the back of the
PCB) making a voltage divider with the 1K input resistor.
The level at point A should be limited to less than 2 Vp-p.

Note: This has not shown up to be a problem with the receiver, so is not needed.

Note: Isolating the display from ground has solved the problem of birdies getting into the receiver. The following method was suggested before the discovery of floating the display from ground.

The following information is left here in case someone might need to use these techniques in a build where they must be used to eliminate the noise from the counter.

The elimination of birdies from the AADE Frequency Counters, or from any counter, needs to be approached from several different angles. Filtering the power supply, shielding the counter, and having a good ground attached to the receiver all need to be considered.

The frequency counters supplied with the kit will have a 20 MHz TXO to raise the running frequency of the counter out of the range of the receiver.

Most of the birdies of the receiver have been found to come from the updating of the LCD display. Sometimes a 10 MHz signal from the 20 MHz TXO will be heard, divided by the counter.

If the kit is built in a case, moving the frequency counter 3 to 6 inches away from the receiver boards will help immensely in keeping birdies from getting into the receiver.

Filter the power supply

The best way to isolate the power supply line to the counter is by the use of a three terminal regulator (78xx series of regulators). Use a 7808 or 7809 just inside the shielding for the counter. Place a .01 bypass ceramic, plus a 100 mfd (or any high value electrolytic) at the input terminal of the regulator.

Solder a 2.2 mfd tantalum or electrolytic at the output terminal of the regulator. Do not place a .01 at the output of the regulator, as this may cause oscillations at the regulator. Higher value electrolytics at the output may be used, but the optimal value is 2.2 mfd.

Shielding

A good shield is absolutely necessary. When the counter is placed right below the First Mixer, as shown in the prototype pictures on this web site, no shielding yields birdies that nail the S-Meter.

A top cover placed directly between the counter and the First Mixer lower the birdies to where there are no S-Meter readings. Picture

Distance from the receiver also helps. When the receiver boards are placed in a box, with the counter placed 3" to 6" away on the front panel, the birdies are much easier to eliminate.

Proper Drive Levels

Driving the frequency counter inputs too strong is a major reason for birdies getting into the receiver.

Using a 3.3pf capacitor right at the outputs of the VFO (Picture) and the Crystal Oscillator (Picture), before any amplification, yielded a drive level that ran the counter and kept birdies at a minimum. A peak to peak reading on an oscilloscope was about .6 volt from both locations.

Coupling capacitors of 3.3pf are used at the "Freq Cntr" connections at both the VFO and Crystal Oscillator.

Grounding the Cables, the PCB board, and both sides of the PCB Shield Box

Two coax cables are used to bring in the signals from the VFO and the Crystal Oscillator. To prevent ground loops which can carry birdies to the receiver, the insulation around the shields of the cables are removed and the shields are grounded on the inside of the case right where they exit. Picture

The PCB board of the Frequency Counter should have a direct lead to ground. Picture

Earlier models of the DFD2 did not have a ground connected to the bezel around the LCD display. Check with a DVM and see if yours is grounded to the PCB ground. The current batch that AADE is shipping are all grounded to the PCB board, but some earlier ones were not.

If your bezel is not grounded, use a small file and file off the chrome on one side where it is least likely to be seen. Then solder a wire from the bezel to the closest ground.

Be sure all the PCB material used to make the box is grounded. The best way to make sure is to solder loops across the PCB material on all the side panels. Picture

Use RF Chokes at 12 Volt input, Switch pins, and Control wires

An RF choke was placed where 12 Volts entered the box going to the 9 volt regulator. A feedthrough capacitor was also used, but may not be necessary. If feedthrough caps are not used, use shielded cable and solder the shield where it enters the box.

RF chokes were used at the DFD2 + and - frequency control pins. The LCD updating noise was found at these pins and the RF chokes eliminates the noise.

A Good Ground

A good earth ground is needed for any of the suppression techniques to work, especially the power supply and the shielding.

A "floating" receiver, one with no ground, cannot effectively utilize the bypass elements used in the receiver, not only for counter birdie elimination, but from signals traveling from the first board to the second.

The receiver has been supplied with a lot of bypass caps and RF chokes to eliminate almost all the problems associated with signals going where they are not supposed to go, but without a good ground, the suppression is not as effective.

Final Note

The high impedance levels of this receiver, and the high gain of the VFO amplifiers make the problem worse with this receiver than with many other receivers using lower impedance levels and less gain in the VFO circuitry.

In most cases, band noise helps cover up a lot of the residual birdies in the receiver. The birdies are not amplified the same amount as band signals. When a band is wide open, or an outdoor resonant antenna is used, there is little problem with the counter birdies.

The most effective solution is to add an on-off switch to the counter, so when absolute freedom from interference is wanted, the counter can simply be shut off, and then turned on momentarily to check your operating frequency.